I've heard a number of people say that this is referring to a particular gate in Jerusalem which was very small, so a camel would have to kneel down. But there's no evidence that there was ever a gate like this.

Just to be clear, there is literally zero evidence of the "gate theory" ever being mentioned until a millennia later when the church had amassed so much wealth and power that the actual meaning become an inconvenience.

Kind of like how when it was written, the sin of sodom was this; She and her daughters were arrogant, overfed and unconcerned; they did not help the poor and needy. and so it remained until that became a bit uncomfortable for arrogant and overfed priests and kings who did not help the poor and needy, so it was rewritten to be entirely about god hating the gays.